Tech Tip: Understanding Multi-Part Part Studios vs. Assemblies in Onshape

Part Studios in Onshape behave a bit differently from most on-premises part files that engineers are used to working with. This adds an additional level of design power and capability, but it often raises questions about when to use Part Studios vs. assemblies.

At first glance, a multi-part Part Studio might look like a basic assembly or subassembly. However, there are important differences between the two environments, and this Tech Tip focuses on when to use each and how they support one another to facilitate your design process.

How Multi-Part Part Studios Work

Onshape fully supports the creation and modification of multiple parts inside of a single Part Studio – but what exactly does this mean?

Onshape’s cloud database architecture eliminates individual part files, allowing multiple distinct parts to be created, tracked, and updated simultaneously within a single Part Studio. This key distinction means that multi-part Part Studios in Onshape are not the same as multi-body part environments in other systems.

A common limitation or complaint about multi-body part design is that it can be difficult to isolate the different bodies as their own entities without establishing a fragile web of file path references back to the original multi-body file.

In Onshape, however, all entities created in a Part Studio are immediately recognized as first-class parts in the database. This means every part has its own identity, part number, and any other properties or metadata you want, as well as being independently revision-controlled. It also means that individual parts from those Part Studios can be inserted independently into drawings and assemblies.

Because of this behavior, it is sometimes not immediately apparent to new users what the key differences are between these multi-part Part Studios and Assemblies. To be abundantly clear, multi-part Part Studios do not and were never intended to replace Assemblies, as both are almost always necessary for full design projects.

When to Use Multi-Part Part Studios

The first question you might be asking yourself is, why might you even want to take advantage of these multi-part Part Studios in the first place?

Multi-part design is a powerful tool that doesn’t fall to the usual setbacks of multi-body environments, as discussed earlier. The primary benefit of multi-part design is being able to reuse common components like sketches, existing faces/edges, dimensions, or variables across multiple parts. The best way to clarify when to use them is to address a few key times when you really don’t want to put multiple parts inside a single Part Studio.

The most common misuse of a multi-part Part Studio is bundling unrelated parts into a single Part Studio for convenience. Following Onshape best practices, these are better off in their own Part Studio to keep each Feature list short and intentional, which keeps individual documents fast and easy to navigate as a project grows. Keep this in mind, along with other document performance tips, to keep performance snappy and responsive.

Another time to consider avoiding putting multiple parts in one Part Studio is when the parts will be commonly reused across multiple subsystems or entirely separate projects. While you certainly can insert individual parts from multi-part Part Studios, it can be much easier from a data management standpoint to simply keep common parts and components separate in their own environments.

So, when should you use a multi-part Part Studio?

In essence, if editing one part's sketch or feature should logically affect the others (i.e., a shared parting line, a common mounting sketch, a family of parts driven by the same variables), a multi-part Part Studio is the right choice. If you're just storing unrelated parts close together for convenience, model them separately and bring them together into an assembly instead.

When to Use Assemblies

Even with all that said, perhaps it’s becoming clear why it isn’t always obvious where the line between assembly and multi-part Part Studio is drawn, but there are two major differences between the two modeling environments.

Part Studio environments are purely for modeling your part geometry, with all parts modeled statically relative to the origin. The opposite is true for assemblies – you add the intended relationships between components, including movement, here rather than define the features that build your part.

Thus, the first reason to use assemblies is that you need to define the spatial relationships of your components. When you bring an entire multi-part Part Studio into an assembly, the parts come in in their correct relative location, as shown below. You have the option to keep them in the same positions relative to one another or to move them independently, depending on your needs.

Bonus Tip: If all elements of the Part Studio are static with respect to one another and are already in the correct position, you can save time and either insert them as a rigid Part Studio or group them.

The second element, often less obvious to new users, is extremely important to understand. All part instancing should happen in the assembly, not the Part Studio.

A Part Studio treats each part in its Parts list as a unique instance. If you copy, pattern, or mirror a part inside a Part Studio, Onshape recognizes these components as individual instances, even if they are geometrically identical. Things may look okay in the Part Studio, but once you get to your assembly, the BOM will show multiple quantities of 1, rather than the number of instances you intended to create, as shown below.

As you might expect, then, Assemblies are the correct way to handle this. When you insert the same part multiple times, or otherwise pattern/instance it into an assembly, Onshape recognizes them as instances of a single part and rolls them up as the correct quantity on the BOM line. This is the mechanism that makes Onshape's BOM accurate.

Instancing also improves Part Studio regeneration time and overall document performance. This is especially useful as part geometry grows in complexity or the total quantity of parts increases, as assemblies process instancing much better.

The Takeaway for New Users

Part Studios are where you define what a part is: its shape, features, and its relationship to sibling parts that share design intent. Assemblies are where you define how parts relate and move, and how many of each you actually need for the BOM. The distinction may seem simple now, but mastering this early can be a key step toward unlocking some of the new workflows that so many users make the switch to Onshape for.

This Tech Tip helped you learn the major differences between multi-part Part Studios and assemblies and some examples of when to (and when not to) use them. The natural next step from here would be to dive deeper into how in-context Part Studios let you work with Assembly contexts, or how to use the Derived tool in your Part Studios to take top-down modeling to the next level.